Method for making a lithographic printing plate with frequency modulated halftone dots

ABSTRACT

There is provided a method for making a lithographic printing plate according to the DTR-process from a printing plate precursor comprising on a hydrophilic surface of a support in the order given a layer of physical development nuclei and a silver halide emulsion layer from an original containing continuous tones comprising the steps of: 
     frequency modulation screening said original to obtain screened data, 
     image-wise exposing said printing plate precursor according to said screened data, 
     developing a thus obtained image-wise exposed lithographic printing plate precursor by means of an alkaline processing solution in the presence of a developing agent and a silver halide solvent yielding a lithographic printing plate, characterized in that the frequency modulated halftone dot with the greatest main dimension on said lithographic printing plate has a main dimension of more than 21 μm.

FIELD OF THE INVENTION

The present invention relates to a method for making a lithographicprinting plate according to the silver salt diffusion transfer processcomprising a frequency modulation screening of the original.

BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversalprocess, hereinafter called DTR-process, have been described e.g. inU.S. Pat. No. 2,352,014 and in the book "Photographic Silver HalideDiffusion Processes" by Andre Rott and Edith Weyde--The Focal PressLondon--and New York, (1972).

In the DTR-process non-developed silver halide of an information-wiseexposed photographic silver halide emulsion layer material istransformed with a so-called silver halide solvent into soluble silvercomplex compounds which are allowed to diffuse into an image receivingelement and are reduced therein with a developing agent, generally inthe presence of physical development nuclei, to form a silver imagehaving reversed image density values ("DTR-image") with respect to theblack silver image obtained in the exposed areas of the photographicmaterial.

A DTR-image bearing material can be used as a planographic printingplate wherein the DTR-silver image areas form the water-repellentink-receptive areas on a water-receptive ink-repellent background.

The DTR-image can be formed in the image receiving layer of a sheet orweb material which is a separate element with respect to thephotographic silver halide emulsion material (a so-called two-sheet DTRelement) or in the image receiving layer of a so-calledsingle-support-element, also called mono-sheet element, which containsat least one photographic silver halide emulsion layer integral with animage receiving layer in waterpermeable relationship therewith. It isthe latter mono-sheet version which is preferred for the preparation ofoffset printing plates by the DTR method.

Two types of the mono-sheet DTR offset printing plate exist. Accordingto a first type disclosed in e.g. U.S. Pat. No. 4,722,535 andGB-1,241,661 a support is provided in the order given with a silverhalide emulsion layer and a layer containing physical development nucleiserving as the image-receiving layer. After information-wise exposureand development by means of an alkaline processing solution in thepresence of a developing agent and a silver halide solvent the imagedelement is used as a printing plate without the removal of the emulsionlayer.

According to a second type of mono-sheet DTR offset printing plate ahydrophilic surface of a support is provided in the order given with alayer of physical development nuclei and a silver halide emulsion layer.After information-wise exposure and development by means of an alkalineprocessing solution in the presence of a developing agent and a silverhalide solvent the imaged element is treated to remove the emulsionlayer so that a support carrying a silver image is left which is used asa printing plate. Such type of lithographic printing plate is disclosede.g. in U.S. Pat. No. 3,511,656.

From the above it will be clear that lithographic printing is onlycapable of reproducing two tone values because the areas will accept inkor not. Thus lithographic printing is a so called binary process. Inorder to reproduce originals having continuously changing tone values bysuch process halftone screening techniques are applied.

In a commonly used halftone screening technique, the continuouslychanging tone values of the original are modulated with periodicallychanging tone values of a superimposed two-dimensional screen. Themodulated tone values are then subject to a thresholding process whereintone values above the threshold value will be reproduced and those belowwill not be reproduced. The process of tone-value modulation andthresholding results in a two-dimensional arrangement of equally spaced"screen dots" whose dimensions are proportional to the tone value of theoriginal at that particular location. The number of screen dots per unitdistance determines the screen frequency or screen ruling. Thisscreening technique wherein the screen frequency is constant andinversely proportional to the halftone cell size, is referred to asamplitude-modulation screening or autotypical screening. This techniquecan be implemented photo-mechanically or electronically.

It will further be clear that in order to reproduce a color image usinglithographic printing it will be required to separate the image in threeor more part-images corresponding to primary colors that when printedover each other yield the desired color at any place within the image.Each of these color separations has to be screened as described above.

It is well known that the above described procedure of screening resultsin certain artifacts on a copy obtained in lithographic printing. Suchartifacts are e.g. enlarging of the screen dots on the press, Moirepatterns, color shifts, etc.

A lot of variants of the dot size modulation screening have beendisclosed in order to remedy these artifacts but none of them wascapable of completely eliminating the enlarging of the screen dots onthe press and the Moire patterns and dot frequency modulation screeningtechniques have therefore been suggested to further reduce the problem.

According to frequency modulation screening the distance between thehalftone dots is modulated rather then their size. This technique,although well known in the field of low resolution plain paper printers,has only recently obtained much attention for offset printing and otherhigh end printing methods.

Methods for making a lithographic printing plate according to the silversalt diffusion transfer process comprising the steps of frequencymodulation screening an original to obtain screened data and image-wiseexposing an imaging element according to said screened data, saidexposure being scan-wise and/or said imaging element having a flexiblesupport and/or a photosensitive layer comprising a direct positivesilver halide emulsion have been disclosed in e.g. EP-A 620673, EP-A620674 and EP-A 94201942.3.

When a mono-sheet DTR offset printing plate of the second type isprepared comprising the steps of frequency modulation screening anoriginal to obtain screened data and image-wise exposing a precursor ofa mono-sheet DTR offset printing plate of the second type according tosaid screened data and developing a thus obtained image-wise exposedlithographic printing plate precursor by means of an alkaline processingsolution in the presence of a developing agent and a silver halidesolvent, this printing plate has a constrained tone range in print, arestricted development latitude and a marked loss of the tone value overthe whole tone scale but especially in the lower range of said scale infunction of the number of printed copies when said imaging elements doesnot comprise halftone dots of more than 21 μm.

SUMMARY OF INVENTION

It is an object of the present invention to provide a method for makinga lithographic printing plate according to the DTR-process from aprinting plate precursor comprising on a hydrophilic surface of asupport in the order given a layer of physical development nuclei and asilver halide emulsion layer by means of frequency modulation screeningan original to obtain screened data with improved printing propertiese.g. an extended tone scale in print, a larger development latitude anda smaller loss of the tone value over the whole tone scale butespecially in the lower range of said scale in function of the number ofprinted copies.

Further objects of the present invention will become clear from thedescription hereinafter.

According to the present invention there is provided a method for makinga lithographic printing plate according to the DTR-process from aprinting plate precursor comprising on a hydrophilic surface of asupport in the order given a layer of physical development nuclei and asilver halide emulsion layer from an original containing continuoustones comprising the steps of:

frequency modulation screening said original to obtain screened data,

image-wise exposing said printing plate precursor according to saidscreened data.

developing a thus obtained image-wise exposed lithographic printingplate precursor by means of an alkaline processing solution in thepresence of a developing agent and a silver halide solvent yielding alithographic printing plate, characterized in that the frequencymodulated halftone dot with the greatest main dimension on saidlithographic printing plate has a main dimension of more than 21 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and without theintention to limit the invention thereto by means of the followingdrawing:

The sole FIGURE shows a schematic representation of a circuit forimplementing a halftoning method suitable for use in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Surprisingly, it was found that when a lithographic printing plate isprepared according to the DTR-process from a printing plate precursorcomprising on a hydrophilic surface of a support in the order given alayer of physical development nuclei and a silver halide emulsion layerby means of frequency modulation screening an original to obtainscreened data, this printing plate has improved printing properties e.g.an extended tone range in print, a larger development latitude and asmaller loss of the tone value over the whole tone scale but especiallyin the lower range of said scale in function of the number of printedcopies when the frequency modulated halftone dot with the greatest maindimension on said lithographic printing plate has a main dimension ofmore than 21 μm.

A "micro dot" or "elementary dot" or "recorder element" is the smallestaddressable unit on a digital recorder. Its size is referred to as therecorder "pitch" (P). In connection with photographic materials whichwere subjected to a process comprising a frequency modulation screeningof the original a frequency modulated halftone dot is the smallest imageunit that is rendered on said photographic material after exposing andprocessing said material.

The main dimension of a frequency modulated halftone dot according tothe invention is the side when said dot is a square: the length whensaid dot is a rectangle: the diameter when said dot is a circle; thelongest axis when said dot is an ellipse and the square root of thesurface area when said dot has another form.

The main dimension of a frequency modulated halftone dot with thegreatest main dimension on said printing plate according to theinvention is more than 21 μm, preferably more than 25 μm. The maindimension of a frequency modulated halftone dot with the greatest maindimension on said printing plate according to the invention is dependenton the application wherein said printing plate is used but is preferablynot more than 100 μm.

Preferably the main dimension of at least 75% of the frequency modulatedhalftone dots, more preferably of at least 90% of the frequencymodulated halftone dots, still more preferably of substantially all ofthe frequency modulated halftone dots on said printing plate is morethan 21 μm. Still more preferably the main dimension of at least 75% ofthe frequency modulated halftone dots, even more preferably of at least90% of the frequency modulated halftone dots, particularly preferably ofsubstantially all of the frequency modulated halftone dots on saidprinting plate is more than 25 μm.

In the case of the present invention the dimension of a frequencymodulated halftone dot can be most easily measured in the areas of lowdensity on the black frequency modulated halftone dots and in the areasof high density on the shining frequency modulated halftone dots

A frequency modulated halftone dot with the greatest main dimensionaccording to the invention can consist of one microdot and is thenobtained by an optical and/or mechanical manipulation of the recordingbeam of the digital image recorder. Preferably said halftone dot consistof a m * n matrix of "micro dots" wherein m represents 2 or a greaterinteger and n represents 1 or a greater integer and m * P and/or n *P>21 μm. Although m and n can have different values they are preferablyequal, yielding a halftone dot that is essentially a square.

The original containing continuous tones is screened by frequencymodulation screening. Frequency modulation screening is a technique inwhich the continuously changing tone values of an original arereproduced by means of equally sized micro dots, the number of which isproportional to the tone value of the original image. The name frequencymodulation refers to the fact that the number of micro dots per unitsurface (the frequency) fluctuates in proportion to the tone value inthat same area.

Various dot frequency modulation screening techniques suitable for usein connection with the present invention have been disclosed and theycan be divided into the following subclasses:

1) Point to Point thresholding based techniques,

2) Error Diffusion along a line by line, column by column scan (andvariations).

3) Error Propagation along a Hilbert scan (and variations).

4) Special techniques.

A special technique is disclosed in DE 2,931,092, and further developedin U.S. Pat. No. 4,485,397.

The most representative technique of point to point thresholding is thehalftoning based on the "Bayer" dither matrix as described by Bayer, B.E., "An optimum method for two level rendition of continuous-tonepictures", Proc. IEEE International Conference on Communications,Conference Record, pp. (26-11), (26-15)., 1973. This Bayer dither matrixhas a size that is a power of two, and contains threshold values thatare arranged in such a fashion that, when thresholded against increasinglevels of density, every halftone dot is "as far away as possible" fromthe halftone dots that are used to render the lower density levels.

A number of variations on this technique are developed. A method isdisclosed in EP-A 639023 wherein said frequency modulation screening ofan original comprises the steps of : (i) generating a non-halftonevalue: (ii) altering said non-halftone value at a higher tone resolutionthan the tone resolution of said original and (iii) utilizing saidaltered non-halftone value to produce a halftone value for the frequencymodulated halftone screening.

Another method is disclosed in EP-A 94202329.2 wherein said frequencymodulation screening of an original originates a halftone screen havinga plurality of halftone dots, at least some of which have a size greaterthan the size of a pixel of said contone original and at least some ofwhich have a size equal to the size of a pixel of said contone original.Said methods may be used alone or in combination.

Another Point to Point thresholding technique uses a "Blue Noise Mask"instead of a Bayer dither matrix. It is described in U.S. Pat. No.5,111,310. The Blue Noise Mask is the result of an optimization(filtering) performed iteratively (for the subsequent threshold"layers") on the Fourier transform of the threshold matrix.

Another frequency modulation screening techniques suitable for use inconnection with the present invention is the well known Error diffusionfirst described by Floyd and Steinberg "An adaptive algorithm forspatial grey scale" SID 75 Digest. Society for information display 1975,pp. 36-37. According to the error diffusion technique the image pixelsof a continuous tone image are processed one after the other accordingto a predetermined path e.g. from left to right and top to bottom.

The tone value of each image pixel is thereby compared with a thresholdvalue which is generally the tone value half-way the tone scale e.g. 128when the tones of the image-pixels range from 0 to 256. Depending onwhether the tone value of the image pixel is above or below thethreshold value a halftone dot will be set or not in the correspondingreproduction of the image pixel. The resulting error or weighted error,i.e. the difference between the reproduction value and actual value ofthe image pixel, is then added to the tone value of one or moreneighbouring image pixels that are still unprocessed. Details about theerror diffusion screening method may be found in the aforementionedreference or in U.S. Pat. No. 5,175,804.

A more preferred variant of frequency modulation screening for use inconnection with the present invention is a method similar to the errordiffusion with the exception that the order in which the image pixelsare processed can be described by a space filling deterministic fractalcurve or a randomized space filling curve.

This type of frequency modulation screening comprises the followingsteps:

selecting an unprocessed image pixel from the original continuous toneimage according to a space filling deterministic fractal curve or arandomized space filling curve and processing said unprocessed imagepixel as follows:

determining from the tone value of said unprocessed image pixel areproduction value to be used for recording said image pixel on arecording medium e.g. a photographic film or lithographic printing plateprecursor.

calculating an error value on the basis of the difference between saidtone value of said unprocessed image pixel and said reproduction value,said unprocessed image pixel thereby becoming a processed image pixel,

adding said error value to the tone value of an unprocessed image pixeland replacing said tone value with the resulting sum or alternativelydistributing said error value over two or more unprocessed image pixelsby replacing the tone value of each of said unprocessed image pixels towhich said error value will be distributed by the sum of the tone valueof the unprocessed image pixel and part of said error,

repeating the above steps until all image pixels are processed.

A suitable deterministic fractal curve is for example the so called"Hilbert Curve" disclosed by Witten Ian H., and Radford M. Neal, "UsingPeano Curves for Bilevel Display of Continuous-Tone Images", IEEE CG&A,May 1982, pp. 47-52.

According to a particularly preferred embodiment in connection with thepresent invention the order of processing the image pixels is ruled by arandomized space filling curve. With the term "randomized space fillingcurve" is meant that the processing of the image pixels followsbasically a pre-determined curve that assures that each image pixel willbe processed but which curve is randomized at a number of points so thatpatterns are avoided.

According to an alternative a randomized space filling curve may beobtained by dividing the image into matrices of image pixels. Withineach of these matrices the image pixels are processed at random untilall image pixels are processed. The order in which the matrices areprocessed may then be selected at random or in a predetermined way.

The most preferred alternative over the above method of dividing theimage into matrices is the recursively division of the image intosmaller matrices until the size of a matrix reaches an image pixel. Atevery subdivision into smaller submatrices a random ordering ofprocessing the matrices is assigned to every submatrix. More details onthese screening techniques are disclosed in e.g. EP-A 571.010 and EP-A93201114.1, which therefor are hereby incorporated by reference.

The figure shows a circuit to perform a frequency modulation screeningin combination with a binary recording device, e.g. an image-setter.First the different building blocks of this circuit are described, lateron its operation will be explained.

Block (20) is a memory block containing the contone pixel values of animage. Typically these are 8 bit values, organized as N lines with Mcolumns. Block (30) is a memory block with the same lay out as block(20), in which the halftoned pixel values will be stored. In the case ofa binary recording device, every halftoned pixel word has a length of 1bit. Block (80) is a device capable of image-wise exposing a substratee.g. a photographic film or a lithographic printing plate precursorusing the information in block (30). Block (70) is an arithmetic unitcapable of calculating the sum of the pixelvalue P(i,j) and the error Eat the output of a delay register (60). The conversion of a contonepixel value into a halftoned pixel value takes place in block (40). Thisconversion may be based on a thresholding operation: if the contonevalue at point (i.j) is below the value of 128, a value "0" is stored inthe halftone memory, otherwise a "1" is stored. Block (50) contains anarithmetic unit that is capable to calculate the error between theoriginal contone value, and the halftoned pixel value, and to store itin the delay register (60). Block (8) is a counter that sequences theprocessing of the N*M pixels of the image. Block (10) is LUT with N*Mentries (one for every image pixel), and a UNIQUE combination of a rowand column address that corresponds with one pixel position in theimage. Block (5) is a clock.

The table of block (10) thus holds the order in which the image pixelswill be processed. This table may be calculated according to one of themethods described above.

The operation of the diagram is now explained. At every clock pulse, thecounter (8) is incremented, and a new pair of coordinates (i(n),j (n))is obtained from block (10). These coordinates are used as addressvalues to the pixel memory (20), to obtain a contone pixel valueP(i(n),j(n)). This pixel value is immediately added to the errorE(i(n-1),j(n-1)), that was stored in register (60) after the previoushalftone step, and the sum of both is compared to the threshold value(41) in block (40). The outcome of the thresholding operation determinesthe value H(i(n),j(n)) that will be written into the halftone pixelmemory at position (i(n),j(n)). At the same time a new errorE(i(n),j(n)) is calculated from the difference between P(i(n).j (n)) andH(i(n),j(n)) and stored in the delay register (60). The circuit isinitialized by setting the counter (8) to 1, the error to 128, and theoperation is terminated when the counter reaches the level N*M. Afterthat, the halftone memory (30) is read out line by line, column bycolumn, and its contents are recorded on a substrate by the recorder(80).

According to a variant of the above circuit the error that is obtainedfrom the difference between the contone pixel and the halftoned pixelvalue, may, instead of being diffused only to the next pixel in theorder of processing, diffused to more than one of the unprocessedpixels. Instead of using the error of one pixel, one may also use anaverage error of a number of pixels.

In case of a color image, the above described screening process isperformed on each of the color separations of the image.

Preferably the color image is separated in its Yellow, Magenta, Cyan andBlack components. Each of these components may then be screened and usedto image-wise expose four lithographic printing plate precursorsaccording to the present invention. Four lithographic printing plates,one for each color separation, will thus be obtained. The colorseparations can then be printed over each other in register in alithographic printing machine using the four plates.

According to a preferred embodiment of the present invention the CMYKcolor separations are prepared starting from a device independentrepresentation of the color image. More details on this technique aredisclosed in e.g. WO 94/06242, which therefor is incorporated hereby forreference.

Image-wise exposure in accordance with the present invention may proceedby a scan-wise exposure by means of e.g. a laser or LED directlyaccording to said screened data on the printing plate precursor (socalled computer to plate) or it may be performed by first exposingaccording to said screened data an intermediate photographic film ofhigh contrast, generally a high contrast silver halide film, and thenusing the imaged photographic film as a mask for exposing a lithographicprinting plate precursor to a conventional light source in a cameraexposure or contact exposure.

Suitable devices for said scan-wise exposure are e.g. Cathode Ray Tubes,LED's or, most preferably lasers.

Examples of lasers that can be used in connection with the presentinvention are e.g. He/Ne lasers, Argon ion lasers, semiconductor lasers,YAG lasers e.g. Nd-YAG lasers etc..

Photographic films and printing plate precursors having a flexiblesupport are preferably exposed using a drum scanner. Examples of suchexposure units containing a HeNe laser are the image-setters LINOTRONIC300, marketed by LINOTYPE-HELL Co, and Select 5000/7000, marketed byMiles Inc. Such an image-setter provided with an Ar ion laser that canbe used is LS 210, marketed by Dr-Ing RUDOLF HELL GmbH. Such exposureunits provided with a laserdiode that can be used are LINOTRONIC 200,marketed by LINOTYPE-HELL Co, and ACCUSET marketed by Miles Inc.

Because of the stiffness of an aluminium support this type of imagingelements used to be exposed using a flat-bed scanner but recently alsoappropriate drum scanners are used.

A lithographic printing plate precursor suitable for use in the presentinvention comprises in the order given on a hydrophilic surface of asupport a layer of physical development nuclei and a silver halideemulsion layer in water permeable contact with said image receivinglayer.

Layers being in waterpermeable contact with each other are layers thatare contiguous to each other or only separated from each other by (a)waterpermeable layer(s). The nature of a waterpermeable layer is suchthat it does not substantially inhibit or restrain the diffusion ofwater or of compounds contained in an aqueous solution e.g. developingagents or the complexed silver.

The imaging element is preferably prepared by coating the differentlayers on a hydrophilic surface of a support. Alternatively thedifferent layers may be laminated to said image receiving layer from atemporary base holding the layers in reverse order as disclosed in U.S.Pat. No. 5,068,165.

Said hydrophilic surface of a support can be a hardened hydrophiliclayer, containing a hydrophilic binder and a hardening agent coated on aflexible support.

Such hydrophilic binders are disclosed in e.g. EP-A 450,199, whichtherefor is incorporated herein by reference. Preferred hardenedhydrophilic layers comprise partially modified dextrans or pullulanhardened with an aldehyde as disclosed in e.g. EP-A 514,990 whichtherefor is incorporated herein by reference. More preferred hydrophiliclayers are layers of polyvinyl alcohol hardened with a tetraalkylorthosilicate and preferably containing SiO₂ and/or TiO₂ wherein theweight ratio between said polyvinylalcohol and said tetraalkylorthosilicate is between 0.5 and 5 as disclosed in e.g. GB-P 1,419,512,FR-P 2,300,354, U.S. Pat. No. 3,971,660. U.S. Pat. No. 4.28,705, EP-A405,016 and EP-A 450,199 which therefor are incorporated herein byreference.

Flexible supports may be opaque or transparent, e.g. a paper support orresin support. When a paper support is used preference is given to onecoated at one or both sides with an Alpha-olefin polymer. It is alsopossible to use an organic resin support e.g. poly(ethyleneterephthalate) film or poly-Alpha-olefin films. The thickness of suchorganic resin film is preferably comprised between 0.07 and 0.35 mm.These organic resin supports are preferably coated with a hydrophilicadhesion layer which can contain water insoluble particles such assilica or titanium dioxide. Metal supports e.g. aluminum may also beused.

Said hydrophilic surface of a support is preferably a hydrophilicmetallic support e.g. an aluminium foil.

The aluminum support of the imaging element for use in accordance withthe present invention can be made of pure aluminum or of an aluminumalloy, the aluminum content of which is at least 95%. The thickness ofthe support usually ranges from about 0.13 to about 0.50 mm.

The preparation of aluminum or aluminum alloy foils for lithographicoffset printing comprises the following steps: graining, anodizing, andoptionally sealing of the foil.

Graining and anodization of the foil are necessary to obtain alithographic printing plate that allows to produce high-quality printsin accordance with the present invention. Sealing is not necessary butmay still improve the printing results. Preferably the aluminum foil hasa roughness with a CLA value between 0.2 and 1.5 μm, an anodizationlayer with a thickness between 0.4 and 2.0 μm and is sealed with anaqueous bicarbonate solution.

According to the present invention The roughening of the aluminum foilcan be performed according to the methods well known in the prior art.The surface of the aluminum substrate can be toughened either bymechanical, chemical or electrochemical graining or by a combination ofthese to obtain a satisfactory adhesiveness of a silver halide emulsionlayer to the aluminum support and to provide a good water retentionproperty to the areas that will form the non-printing areas on the platesurface.

The electrochemical graining process is preferred because it can form auniform surface roughness having a large average surface area with avery fine and even grain which is commonly desired when used forlithographic printing plates.

Electrochemical graining can be conducted in a hydrochloric and/ornitric acid containing electrolyte solution using an alternating ordirect current. Other aqueous solutions that can be used in theelectrochemical graining are e.g. acids like HCl, HNO₃, H₂ SO₂, H₃ PO₄,that if desired, contain additionally one or more corrosion inhibitorssuch as Al (NO₃)₃, AlCl₃, boric acid, chromic acid, sulfates, chlorides,nitrates, monoamines, diamines, aldehydes, phosphates, H₂ O₂, etc. . . .

Electrochemical graining in connection with the present invention can beperformed using single-phase and three-phase alternating current. Thevoltage applied to the aluminum plate is preferably 10-35 V. A currentdensity of 3-150 Amp/dm² is employed for 5-240 seconds. The temperatureof the electrolytic graining solution may vary from 5°-50° C.Electrochemical graining is carried out preferably with an alternatingcurrent from 10 Hz to 300 Hz.

The toughening is preferably preceded by a degreasing treatment mainlyfor removing greasy substances from the surface of the aluminum foil.

Therefore the aluminum foil may be subjected to a degreasing treatmentwith a surfactant and/or an aqueous alkaline solution.

Preferably roughening is followed by a chemical etching step using anaqueous solution containing an acid. The chemical etching is preferablycarried out at a temperature of at least 30° C. more preferably at least40° C. and most preferably at least 50° C.

Suitable acids for use in the aqueous etch solution are preferablyinorganic acids and most preferably strong acids. The total amount ofacid in the aqueous etch solution is preferably at least 150 g/l. Theduration of chemical etching is preferably between 3 s and 5 min.

After toughening and optional chemical etching the aluminum foil isanodized which may be carried out as follows.

An electric current is passed through the grained aluminum foil immersedas an anode in a solution containing sulfuric acid, phosphoric acid,oxalic acid, chromic acid or organic acids such as sulfamic,benzosulfonic acid, etc. or mixtures thereof. An electrolyteconcentration from 1 to 70% by weight can be used within a temperaturerange from 0°-70° C. The anodic current density may vary from 1-50 A/dm²and a voltage within the range 1-100 V to obtain an anodized film weightof 1-8 g/m² Al₂ O₃.H₂ O. The anodized aluminum foil may subsequently berinsed with demineralised water within a temperature range of 10°-80° C.

After the anodizing step sealing may be applied to the anodic surface.Sealing of the pores of the aluminum oxide layer formed by anodizationis a technique known to those skilled in the art of aluminumanodization. This technique has been described in e.g. the"Belgisch-Nederlands tijdschrift voor Oppervlaktetechnieken vanmaterialen", 24ste jaargang/januari 1980, under the title"Sealing-kwaliteit en sealing-controle van geanodiseerd Aluminum".Different types of sealing of the porous anodized aluminum surfaceexist.

Preferably, said sealing is performed by treating a grained and anodizedaluminum support with an aqueous solution containing a bicarbonate asdisclosed in EP-A 567178, which therefor is incorporated herein byreference.

Preferably each of the above described steps is separated by a rinsingstep to avoid contamination of the liquid used in a particular step withthat of the preceding step.

To promote the image sharpness and, as a consequence thereof, thesharpness of the final printed copy, the anodization layer may becoloured in the mass with an antihalation dye or pigment e.g. asdescribed in JA-Pu-58-14,797.

Subsequent to the preparation of the hydrophilic layer of a support asdescribed above, said hydrophilic layer may be immediately coated with asolution containing the physical development nuclei or may be coatedwith said solution at a later stage.

The image receiving layer containing physical development nuclei ispreferably free of hydrophilic binder but may comprise small amountse.g. upto 80% by weight of the total weight of said layer of ahydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicityof the surface.

Preferred development nuclei for use in accordance with the presentinvention are sulphides of heavy metals e.g. sulphides of antimony,bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, andzinc. Especially suitable development nuclei in connection with thepresent invention are palladium sulphide nuclei. Other suitabledevelopment nuclei are salts such as e.g. selenides, polyselenides,polysulphides, mercaptans, and tin (II) halides. Heavy metals,preferably silver, gold, platinum, palladium, and mercury can be used incolloidal form.

The photosensitive layer used according to the present invention may beany layer comprising a hydrophilic colloid binder and at least onesilver halide emulsion, at least one of the silver halide emulsionsbeing photosensitive.

The photographic silver halide emulsion(s) used in accordance with thepresent invention can be prepared from soluble silver salts and solublehalides according to different methods as described e.g. by P. Glafkidesin "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G.F. Duffin in "Photographic Emulsion Chemistry",

The Focal Press, London (1966), and by V. L. Zelikman et al in "Makingand Coating Photographic Emulsion", The Focal Press, London (1966).

For use according to the present invention the silver halide emulsion oremulsions preferably consist principally of silver chloride while afraction of silver bromide may be present ranging from 1 mole % to 40mole %. Most preferably a silver halide emulsion containing at least 70mole % of silver chloride is used.

The average size of the silver halide grains may range from 0.10 to 0.70μm, preferably from 0.25 to 0.45 μm.

Preferably during the precipitation stage iridium and/or rhodiumcontaining compounds or a mixture of both are added. The concentrationof these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole ofAgNO₃, preferably between 10⁻⁷ and 10⁻⁵ mole per mole of AgNO₃.

The emulsions can be chemically sensitized e.g. by addingsulphur-containing compounds during the chemical ripening stage e.g.allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Alsoreducing agents e.g. the tin compounds described in BE-P 493,464 and568,687, and polyamines such as diethylene triamine or derivatives ofaminomethane-sulphonic acid can be used as chemical sensitizers. Othersuitable chemical sensitizers are noble metals and noble metal compoundssuch as gold, platinum, palladium, iridium, ruthenium and rhodium. Thismethod of chemical sensitization has been described in the article of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

Apart from negative-working silver halide emulsions that are preferredfor their high photosensitivity, use can be made also of direct-positivesilver halide emulsions that produce a positive silver image in theemulsion layer(s) and a negative image on the image-receiving layer.

Suitable direct positive silver halide emulsions for use in accordancewith the present invention are silver halide emulsions that have beenpreviously fogged or that mainly form an internal latent image.

Internal latent image-type silver halide emulsions that can be used inaccordance with the present invention have been described in e.g. U.S.Pat. Nos. 2,592,250, 3,206,313, 3,271,157, 3,447,927, 3,511,662,3,737,313, 3,761,276, GB-A 1,027,146, and JA Patent Publication No.34,213/77. However, the silver halide emulsions used in the presentinvention are not limited to the silver halide emulsions described inthese documents.

The other type of direct positive type silver halide emulsions for usein accordance with the present invention, which is of the previouslyfogged type, may be prepared by overall exposing a silver halideemulsion to light and/or by chemically fogging a silver halide emulsion.Chemical fog specks may be formed by various methods for chemicalsensitization.

Chemical fogging may be carried out by reduction or by a compound whichis more electropositive than silver e.g. gold salts, platinum salts,iridium salts etc., or a combination of both. Reduction fogging of thesilver halide grains may occur by high pH and/or low pAg silver halideprecipitation or digestion conditions e.g. as described by Wood J. Phot.Sci. 1 (1953), 163 or by treatment with reducing agents e.g. tin(II)salts which include tin(II)chloride, tin complexes and tin chelates of(poly)amino(poly)carboxilic acid type as described in British Patent1,209,050, formaldehyde, hydrazine, hydroxylamine, sulphur compoundse.g. thiourea dioxide, phosphonium salts e.g.tetra(hydroxymethyl)-phosphonium chloride, polyamines e.g.diethylenetriamine, bis(p-aminoethyl)sulphide and its water-solublesalts, hydrazine derivatives, alkali arsenite, amine borane etc. ormixtures thereof.

When fogging of the silver halide grains occurs by means of a reducingagent e.g. thiourea dioxide and a compound of a metal moreelectropositive than silver especially a gold compound, the reducingagent is preferably used initially and the gold compound subsequently.However, the reverse order can be used or both compounds can be usedsimultaneously.

In addition to the above described methods of chemically foggingchemical fogging can be attained by using said fogging agents incombination with a sulphur-containing sensitizer, e.g. sodiumthiosulphate or a thiocyanic acid compound e.g. potassium thiocyanate.

The silver halide emulsions of the DTR-element can be spectrallysensitized according to the spectral emission of the exposure source forwhich the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region includemethine dyes such as those described by F. M. Hamer in "The Cyanine Dyesand Related Compounds", 1964, John Wiley & Sons. Dyes that can be usedfor this purpose include cyanine dyes, merocyanine dyes, complex cyaninedyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyaninedyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes arethose belonging to the cyanine dyes, merocyanine dyes, complexmerocyanine dyes.

In the case of a conventional light source, e.g. tungsten light, a greensensitizing dye is needed. In case of exposure by an argon ion laser ablue sensitizing dye is incorporated. In case of exposure by a red lightemitting source, e.g. a LED or a HeNe laser a red sensitizing dye isused. In case of exposure by a semiconductor laser special spectralsensitizing dyes suited for the near infra-red are required. Suitableinfra-red sensitizing dyes are disclosed in i.a. U.S. Pat. Nos.2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921,3,582,344, 3,623,881 and 3,695,888.

A preferred blue sensitizing dye, green sensitizing dye, red sensitizingdye and infra-red sensitizing dye in connection with the presentinvention are described in EP-A 554,585.

To enhance the sensitivity in the red or near infra-red region use canbe made of so-called supersensitizers in combination with red orinfra-red sensitizing dyes. Suitable supersensitizers are described inResearch Disclosure Vol 289, May 1988, item 28952. The spectralsensitizers can be added to the photographic emulsions in the form of anaqueous solution, a solution in an organic solvent or in the form of adispersion.

The silver halide emulsions may contain the usual emulsion stabilizers.Suitable emulsion stabilizers are azaindenes, preferably tetra- orpenta-azaindenes, especially those substituted with hydroxy or aminogroups. Compounds of this kind have been described by BIRR in Z. Wiss.Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitable emulsionstabilizers are i.a. heterocyclic mercapto compounds.

The silver halide emulsion layers usually contains gelatin ashydrophilic colloid binder. Mixtures of different gelatins withdifferent viscosities can be used to adjust the rheological propertiesof the layer. But instead of or together with gelatin, use can be madeof one or more other natural and/or synthetic hydrophilic colloids, e.g.albumin, casein, zein, polyvinyl alcohol, alginic acids or saltsthereof, cellulose derivatives such as carboxymethyl cellulose, modifiedgelatin, e.g. phthaloyl gelatin etc.

Preferably the gelatin layer(s) is(are) substantially unhardened.Substantially unhardened means that when such gelatin layer is coated ona subbed polyethylene terephtalate film base at a dry thickness of 1.2g/m², dried for 3 days at 57° C. and 35% R.H. and dipped in water of 30°C., said gelatin layer is dissolved for more than 95% by weight within 5minutes.

The silver halide emulsions may contain pH controlling ingredients.Preferably at least one gelatin containing layer is coated at a pH valuenot below the iso-electric point of the gelatin to avoid interactionsbetween said gelatin containing coated layer and the hereafter mentionedoptional intermediate layer. More preferably the gelatin layercontiguous to said intermediate layer is coated at a pH value not belowthe iso-electric point of the gelatin. Most preferably all the gelatincontaining layers are coated at a pH value not below the iso-electricpoint of their gelatin. Other ingredients such as antifogging agents,development accelerators, wetting agents, and hardening agents forgelatin may be present. The silver halide emulsion layer may compriselight-screening dyes that absorb scattering light and thus promote theimage sharpness. Suitable light-absorbing dyes are described in i.a.U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787 and DE-P 2,453,217.

More details about the composition, preparation and coating of silverhalide emulsions suitable for use in accordance with the presentinvention can be found in e.g. Product Licensing index, Vol. 92, Dec.1971, publication 9232, p. 107-109.

Preferably, the imaging element also comprises an intermediate layerbetween the image receiving layer on the hydrophilic surface of asupport and the photosensitive layer(packet) to facilitate the removalof said layer(packet) thereby uncovering the silver image formed in theimage receiving layer by processing the imaging element.

In one embodiment, the intermediate layer is a water-swellableintermediate layer coated at a ratio of 0.01 to 2.0 g/m² and comprisingat least one non-proteinic hydrophilic film-forming polymer e.g.polyvinyl alcohol and optionally comprising an antihalation dye orpigment as disclosed in EP-A-410500.

In another embodiment, the intermediate layer is a layer comprisinghydrophobic polymer beads having an average diameter not lower than 0.2μm and having been prepared by polymerization of at least oneethylenically unsaturated monomer. Preferably, said intermediate layerin dry condition comprises said hydrophobic polymer beads in an amountof up to 80% of its total weight. Further details are disclosed inEP-A-483415.

A supplemental intermediate layer, which may be present between saidsilver halide emulsion containing layer and said water-swellableintermediate layer or said intermediate layer comprising hydrophobicpolymer beads may incorporate one or more ingredients such as i.a.antihalation dyes or pigment, developing agents, silver halide solvents,base precursors, and anticorrosion substances.

When the imaging element is prepared by laminating a layer packetcomprising a photosensitive layer onto the image receiving layer theintermediate layer(s) are provided on the photosensitive layer(s), thewater-swellable intermediate layer or the intermediate layer comprisinghydrophobic polymer beads having an average diameter not lower than 0.2μm and having been prepared by polymerization of at least oneethylenically unsaturated monomer being the upper layer.

To obtain a lithographic printing plate according to the invention anabove described precursor of the mono-sheet DTR offset printing plate ofthe second type is information-wise exposed as described above inaccordance with the present invention. According to the presentinvention the development and diffusion transfer of the information-wiseexposed imaging element in order to form a silver image in saidphotosensitive layer and to allow unreduced silver halide or complexesformed thereof to diffuse image-wise from the photosensitive layer tosaid image receiving layer to produce therein a silver image, areeffected with the aid of an aqueous alkaline solution in the presence of(a) developing agent(s), and (a) silver halide solvent(s). Thedeveloping agent(s) and/or the silver halide solvent(s) can beincorporated in the aqueous alkaline solution and/or in the imagingelement.

Preferably a silver halide solvent in the aqueous alkaline solution isused in an amount between 0.05% by weight and 5% by weight and morepreferably between 0.5% by weight and 2% by weight. The silver halidesolvent, which acts as a complexing agent for silver halide, preferablyis a water-soluble thiosulphate or thiocyanate e.g. sodium, potassium,or ammonium thiosulphate and sodium, potassium, or ammonium thiocyanate.

Further silver halide solvents that can be used in connection with thepresent invention are e.g. sulphite, amines, 2-mercaptobenzoic acid,cyclic imide compounds such as e.g. uracil, 5,5-dialkylhydantoins, alkylsulfones and oxazolidones.

Further silver halide solvents for use in connection with the presentinvention are alkanolamines. Examples of alkanolamines that may be usedin connection with the present invention correspond to the followingformula: ##STR1## wherein X and X' independently represent hydrogen, ahydroxyl group or an amino group, 1 and m represent 0 or integers of 1or more and n represents an integer of 1 or more. Said alkanolamines maybe present in the alkaline processing liquid in a concentrationpreferably between 0.1% and 5% by weight. However part or all of thealkanolamine can be present in one or more layers of the imagingelement.

Still other preferred further silver halide solvents for use inconnection with the present invention are thioethers. Preferably usedthioethers correspond to the following general formula:

    Z-(R.sup.1 -S).sub.t -R.sup.2 -S-R.sup.3 -y

wherein Z and Y each independently represents hydrogen, an alkyl group,an amino group, an ammonium group, a hydroxyl, a sulfo group, acarboxyl, an aminocarbonyl or an aminosulfonyl, R¹, R² and R³ eachindependently represents an alkylene that may be substituted andoptionally contain a oxygen bridge and t represents an integer from 0 to10. Examples of thioether compounds corresponding to the above formulaare disclosed in e.g. U.S. Pat. No. 4.960.683 and EP-A 554,585.

Still further suitable silver halide solvents are1,2,4-triazolium-3-thiolates, preferably 1,2,4-triazolium-3-thiolatessubstituted with at least one substituent selected from the groupconsisting of a C₁ -C₈ alkyl group that contains at least 3 fluorineatoms, a C₄ -C₁₀ hydrocarbon group and a 4-amino group substituted witha C₁ -C₈ alkyl group that contains at least 3 fluorine atoms and/or a C₄-C₁₀ hydrocarbon group.

Combinations of different silver halide solvents can be used and it isalso possible to incorporate at least one silver halide solvent into asuitable layer of the imaging element and to add at least one othersilver halide solvent to the developing solution.

The alkaline processing liquid may also contain (a) developing agent(s).In this case the alkaline processing liquid is called a developer. Onthe other hand some or all of the developing agent(s) may be present inone or more layers of the photographic material or imaging element. Whenall of the developing agents are contained in the imaging element thealkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the presentinvention are preferably of the p-dihydroxybenzene type, e.g.hydroquinone, methylhydroquinone or chlorohydroquinone, preferably incombination with an auxiliary developing agent being a1-phenyl-3-pyrazolidone-type developing agent and/orp-monomethylaminophenol. Particularly useful auxiliary developing agentsare the 1-phenyl-3-pyrazolidones. Even more preferred, particularly whenthey are incorporated into the photographic material are1-phenyl-3-pyrazolidones of which the aqueous solubility is increased bya hydrophilic substituent such as e.g. hydroxy, amino, carboxylic acidgroup, sulphonic acid group etc.. Examples of 1-phenyl-3-pyrazolidonessubstituted with one or more hydrophilic groups are e.g.1-phenyl-4,4-dimethyl-2-hydroxy-3-pyrazolidone,1-(4-carboxyphenyl)-4,4-dimethyl-3-pyrazolidone etc.. However otherdeveloping agents can be used.

Preferred amounts of the hydroquinone-type developing agents are in therange of 0.05 mole to 0.40 mole per liter and preferred amounts ofsecondary developing agent(s) in the range of 1.8×10⁻³ to 2.0×10⁻¹ moleper liter.

The aqueous alkaline solution in accordance with the present inventionmay further comprise sulphite e.g. sodium sulphite in an amount rangingfrom 40 g to 180 g per liter, preferably from 60 to 160 g per liter incombination with another silver halide solvent.

The quantitative ranges given for the developing agents, silver halidesolvents, and sulphite apply to the amount of these compounds present assolutes in the aqueous alkaline solution during the DTR-processing,whether these compounds make part of the aqueous alkaline solution orwere dissolved from the layers containing them upon application theretoof the aqueous alkaline solution.

The aqueous alkaline solution suitable for use according to the presentinvention preferably comprises aluminum ions in an amount of at least0.3 g/l, more preferably in an amount of at least 0.6 g/l in order toprevent sticking of the emulsion layer to the transporting rollers whenthe emulsion is swollen with the aqueous alkaline solution.

The alkaline processing liquid preferably has a pH between 9 and 14 andmore preferably between 10 and 13, but depends on the type of silverhalide emulsion material to be developed, intended development time, andprocessing temperature.

The processing conditions such as temperature and time may vary withinbroad ranges provided the mechanical strength of the materials to beprocessed is not adversely influenced and no decomposition takes place.

The pH of the alkaline processing liquid may be established by anorganic or inorganic alkaline substance or a combination thereof.Suitable inorganic alkaline substances are e.g. hydroxides of sodium andpotassium, alkali metal salts of phosphoric acid and/or silicic acide.g. trisodium phosphate, orthosilicates, metasilicates,hydrodisilicates of sodium or potassium, and sodium carbonate etc..Suitable organic alkaline substances are e.g. alkanolamines. In thelatter case the alkanolamines will provide or help providing the pH andserve as a silver halide complexing agent.

The aqueous alkaline solution may further comprise hydrophobizing agentsfor improving the hydrophobicity of the silver image obtained in theimage receiving layer. Generally these compounds contain a mercaptogroup or thiolate group and one or more hydrophobic substituents.Particularly preferred hydrophobizing agents aremercapto-1,3,4-thiadiazoles as described in DE-A 1,228,927 and in U.S.Pat. No. 4,563,410, 2-mercapto-5-heptyl-oxa-3,4-diazole and long chain(at least 5 carbon atoms) alkyl substituted mercaptotetrazoles. Thehydrophobizing agents can be used alone or in combination with eachother.

These hydrophobizing compounds can be added to the aqueous alkalinesolution in an amount of preferably 0.1 to 3 g per liter and preferablyin admixture with 1-phenyl-5-mercaptotetrazole, the latter compound maybe used in amounts of e.g. 50 mg to 1.2 g per liter of solution, whichmay contain a minor amount of ethanol to improve the dissolution of saidcompounds.

The aqueous alkaline solution may comprise other ingredients such ase.g. oxidation preservatives, calcium-sequestering compounds,anti-sludge agents, and hardeners including latent hardeners.

Regeneration of the aqueous alkaline solution according to known methodsis, of course, possible, whether the solution incorporates developingagent(s) and/or silver halide solvent(s) or not.

The development may be stopped--though this is often not necessary--witha so-called stabilization liquid, which actually is an acidic stop-bathhaving a pH preferably in the range from 5 to 7.

Buffered stop bath compositions comprising a mixture of sodiumdihydrogen orthophosphate and disodium hydrogen orthophosphate andhaving a pH in said range are preferred.

The development and diffusion transfer can be initiated in differentways e.g. by rubbing with a roller, by wiping with an absorbent meanse.g. with a plug of cotton or sponge, or by dipping the material to betreated in the liquid composition. Preferably, they proceed in anautomatically operated apparatus. They are normally carried out at atemperature in the range of 18° C. to 30° C. and in a time from 5 s to 5min.

After formation of the silver image on the hydrophilic surface of asupport an excess of aqueous alkaline solution still present on the basemay be eliminated, preferably by guiding the foil through a pair ofsqueezing rollers.

The silver image thus obtained in the layer of physical developmentnuclei is subsequently uncovered by treating the imaging element toremove all the layers above the layer containing physical developmentnuclei, thereby exposing the imaged surface of the hydrophilic support.

According to a particularly preferred embodiment of the presentinvention the silver image in the layer of physical development nucleiis uncovered by washing off all the layers above the layer containingphysical development nuclei with rinsing water.

The temperature of the rinsing water may be varied widely but ispreferably between 30° C. and 50° C., more preferably between 35° C. and45° C.

The imaged surface of the hydrophilic surface of a support can besubjected to a chemical treatment that increases the hydrophilicity ofthe non-silver image parts and the oleophilicity of the silver image

This chemical after-treatment is preferably carried out with alithographic composition often called finisher comprising at least onecompound enhancing the ink-receptivity and/or lacquer-receptivity of thesilver image and at least one compound that improves the ink-repellingcharacteristics of the hydrophilic surface.

Suitable ingredients for the finisher are e.g. organic compoundscontaining a mercapto group such as the hydrophobizing compoundsreferred to hereinbefore for the alkaline solution. Preferred compoundscorrespond to one of the following formulas: ##STR2## wherein R⁵represents hydrogen or an acyl group. R⁴ represents alkyl, aryl oraralkyl. Most preferably used compounds are compounds according to oneof the above formulas wherein R⁴ represents an alkyl containing 3 to 16C-atoms. Said (a) hydrophobizing agent(s) is(are) comprised in thefinisher preferably in a total concentration between 0.1 g/l and 10 g/lmore preferably in a total concentration between 0.3 g/l and 3 g/l.

Additives improving the oleophilic ink-repellency of the hydrophilicsurface areas are e.g. carbohydrates such as acidic polysaccharides likegum arabic, carboxymethylcellulose, sodium alginate, propylene glycolester of alginic acid, hydroxyethyl starch, dextrin,hydroxyethylcellulose, polyvinyl pyrrolidone, polystyrene sulphonicacid, polyglycols being the reaction products of ethyleneoxide and/orpropyleneoxide with water or an alcohol and polyvinyl alcohol.Optionally, hygroscopic substances e.g. sorbitol, glycerol,tri(hydroxyethyl)ester of glycerol, and turkey red oil may be added.

Furthermore (a) surface-active compound(s) is preferably also added tothe finisher. The concentration thereof may vary within broad rangesprovided the finisher shows no excessive degree of foaming when platesare finished. Preferred surface-active compound are anionic or non-ionicsurface-active compound.

A suitable finisher as disclosed in U.S. Pat. No. 4,563,410 is acomposition comprising a solution of a mercaptotriazole in a solution ofpolyethylene oxide with a molecular weight of 4,000. Further suitablefinishers have been described in i.a. U.S. Pat. No. 4.062,682.

At the moment the treatment with the finisher is started the surfacecarrying the silver pattern may be in dry or wet state. In general, thetreatment with the finisher does not take long, usually not longer thanabout 30 seconds and it may be carried out immediately after theprocessing and uncovering steps, preferably at a temperature of thefinisher in the range from 30° C. to 60° C.

The finisher can be applied in different ways such as by rubbing with aroller, by wiping with an absorbent means e.g. with a plug of cotton orsponge, or by dipping the material to be treated in the finisher. Theimage-hydrophobizing step of the printing plate may also proceedautomatically by conducting the printing plate through a device having anarrow channel filled with the finisher and conveying the printing plateat the end of the channel between two squeezing rollers removing theexcess of liquid.

As soon as the hydrophilic surface of a support carrying the silverimage has been treated with the finisher, it is ready to be used as aprinting plate.

The following example illustrates the present invention without however,limiting it thereto. All parts, percentages and ratios are by weightunless otherwise indicated.

EXAMPLE 1 (Comparative Example)

Preparation of the lithographic plate precursor.

A 0.30 mm thick aluminium foil (AA 1050) was degreased by immersing thefoil in an aqueous solution containing 10% phosphoric acid andsubsequently etched in an aqueous solution containing 2 g/l of sodiumhydroxide. The foil was then electrochemically grained using analternating current in an aqueous solution containing 4 g/l ofhydrochloric acid and 4 g/l of hydroboric acid at a temperature of 35°C. to form a surface topography with an average center-line roughness Raof 0,6 μm. The aluminium plate was then desmutted with an aqueoussolution containing 30% of sulfuric acid at 60° C. for 120 seconds. Thefoil was subsequently subjected to anodic oxidation in a 20% sulfuricacid aqueous solution to form an anodic oxidation film of 3.0 g/m² ofAl₂ O₃.H₂ O, treated with an aqueous solution containing 20 g/l ofNaHCO₃ at 45° C. for 30 sec and then rinsed with demineralised water anddried.

The imaging element was obtained by coating the grained, anodized andsealed aluminium support with a silver-receptive stratum containing 1.1mg/m² PdS as physical development nuclei.

An intermediate layer was then provided on the dry silver-receptivestratum from an aqueous composition in such a way that the resultingdried layer had a weight of 0.5 g of polymethyl methacrylate beads perm², said composition comprising:

    ______________________________________                                        a 20 % dispersion of polymethyl methacrylate beads                                                        50     ml                                         in a mixture of equal volumes of water and ethanol                            having an average diameter of 1.0 μm                                       Helioechtpapierrot BL (trade mark for a dye sold by                                                       2.5    g                                          BAYER AG, D-5090 Leverkusen, West-Germany)                                    saponine                    2.5    g                                          sodium oleylmethyltauride   1.25   g                                          demineralized water         300    ml                                         ______________________________________                                    

(pH-value : 5.6)

Finally a substantially unhardened photosensitive negative-workingcadmium-free gelatin silver chlorobromoiodide emulsion layer(97.98/2/0.02 mol %) containing 1 mmole/mole AgX of4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene was coated on theintermediate layer, the silver halide being provided in an amountcorresponding to 2.40 g of silver nitrate per m² and the gelatin contentof the emulsion layer being 1.58 g/m², consisting of 0.7 g/m² of agelatin with a viscosity of 21 mPa.s and the remainder of a gelatin witha viscosity of 14 mPa.s.

Preparation of the masks

An original was screened at 2400 dpi according to a frequency modulationscreening wherein the image pixels were processed in a random order byrecursively subdividing the image of the original into matrices untilthe size of a matrix matched the size of an image pixel. The error foreach image pixel was added to the tone value of the next image pixelprocessed. A high contrast silver halide intermediate film was scan-wiseexposed in accordance with the obtained screened data with an imagingbeam having a "pitch" of 10.4 μm. The obtained microdots are arranged ina 2 * 2 matrix giving a frequency modulated halftone dot with a size ofabout 21 μm. Said exposed intermediate film was subjected to a classicalprocessing and was then used as a mask 1. A mask 2 was prepared in anidentical way as mask 1 with the exception that during the scan-wiseexposure of said intermediate film the obtained microdots are arrangedin a 3 * 3 matrix giving a frequency modulated halftone dot with a sizeof about 31 μm.

Preparation of the printing plates.

In order to obtain printing plate I a sample of the printing plateprecursor was exposed through mask 1 to a Theimer light closet with apoint light source in such a way that the 7 μm positive microlines of anUGRA test strip are reproduced on the plate. This is the criterium forthe exposure of a printing plate as is explained in the manual "AgfaCristal Raster User Guide. The exposed printing plate precursor is thenimmersed for 8 s at 24° C. in a freshly made developing solution havingthe following ingredients:

    ______________________________________                                        carboxymethylcellulose      4      g                                          sodium hydroxide            22.5   g                                          anhydrous sodium sulphite   120    g                                          hydroquinone                20     g                                          1-phenyl-3-pyrazolidinone   6      g                                          potassium bromide           0.75   g                                          anhydrous sodium thiosulphate                                                                             8      g                                          ethylene diamine tetraacetic acid tetrasodium salt                                                        2      g                                          demineralized water to make 1000   ml                                         pH (24° C.) = 13                                                       ______________________________________                                    

The initiated diffusion transfer was allowed to continue for 30 s toform a silver image in the image receiving layer.

To remove the developed silver halide emulsion layer and theintermediate layer from the aluminium foil the developed monosheet DTRmaterial was rinsed for 10 s with a water jet at 30° C.

Next, the imaged surface of the aluminium foil was rubbed with afinisher to enhance the water-receptivity of the non-image areas and tomake the image areas oleophilic ink-receptive. The finisher had thefollowing composition:

    ______________________________________                                        10% aqueous n-hexadecyl trimethyl                                                                         25     ml                                         ammonium chloride                                                             20% aqueous solution of polystyrene sulphonic acid                                                        100    ml                                         potassium nitrate           12.5   g                                          citric acid                 20.0   g                                          1-phenyl-5-mercaptotetrazole                                                                              2.0    g                                          sodium hydroxide            5.5    g                                          water to make               1000   ml                                         pH (20° C.) = 4                                                        ______________________________________                                    

A printing plate II was prepared in an identical way as printing plate Iexcept that a sample of the lithographic printing plate precursor wasexposed through mask 2.

A printing plate III was prepared in an identical way as printing plateI except that the exposed sample of the lithographic printing plateprecursor was processed in an developing solution in normal workingconditions (i.e. after developing 5 m² of the lithographic printingplate precursor).

A printing plate IV was prepared in an identical way as printing plateII except That the exposed sample of the lithographic printing plateprecursor was processed in an developing solution in normal workingconditions (i.e. after developing 5 m² of the lithographic printingplate precursor).

The size of the dots on the printing plates was measured. The dots onthe printing plates I and III had a size of about 21 μm, while the dotson the printing plates II and IV had a size of about 31 μm.

The so obtained four plates were printed on a Heidelberg GTO 52 printingpress equipped with a Dahlgren dampener. Hartmann S6920 ink was used andRotamatic 100% was the fountain solution used. Prints were made onZanders Ikonorex 115 g/m² paper at a speed of 5000 prints per hour. Adensity of 1.85 (FOGRA standard) on solid areas was maintained (GretagD186). Densities were measured with a Macbeth RD918 densitometeraccording to the Murray-Davies formula.

After about 200 prints, when good quality prints were obtained, thefollowing results were observed:

a) With printing plate I only from 7% to 95% of the tone scale of themask could be reproduced on the printed copies, where with printingplate II from 2% to 98% of the tone scale of the mask could bereproduced on the printed copies.

b) At 40% of the tone value of the mask 1 the tone value deceased from57% tone value for a copie printed from printing plate I to 45% for acopie printed from printing plate III (a loss of 12%), where at 40% ofthe tone value of the mask 2 the tone value deceased from 58% tone valuefor a copie printed from printing plate II to 56% for a copie printedfrom printing plate IV (a loss of 2%).

The tone value of copies from the printing plate I at 20% tone value ofthe mask I decreased from 35% tone value at the 100^(th) copie to 21% atthe 10,000^(th) copie (a loss of 14%) where the tone value of copiesfrom the printing plate II at a 20% tone value of the mask 2 decreasedfrom 50% tone value at the 100^(th) copie to 45% at the 10,000^(th)copie (a loss of 5%).

So, it is clear from these results that the printing plate II obtainedaccording to the DTR-process from a printing plate precursor comprisingon a hydrophilic surface of a support in the order given a layer ofphysical development nuclei and a silver halide emulsion layer by meansof frequency modulation screening an original to obtain screened data bymeans of frequency modulation and having frequency modulated dots with asize of 31 μm (printing plate according to the invention) has anextended tone range in print, a larger development latitude and asmaller loss of tone value over the whole tone scale but especially inthe lower range of said tone scale in function of the number of printedcopies compared with an identical printing plate I but having frequencymodulated dots with a size of 21 μm (comparison printing plate).

I claim:
 1. A method for making a lithographic printing plate accordingto the DTR-process from a printing plate precursor comprising on ahydrophilic surface of a support in the order given a layer of physicaldevelopment nuclei and a silver halide emulsion layer from an originalcontaining continuous tones comprising the steps of:frequency modulationscreening said original to obtain screened data, image-wise exposingsaid printing plate precursor according to said screened data,developing a thus obtained image-wise exposed lithographic printingplate precursor by means of an alkaline processing solution in thepresence of a developing agent and a silver halide solvent yielding alithographic printing plate with frequently modulated half tone dotswherein at least 75% of the frequency modulated halftone dot on saidlithographic printing plate have a main dimension of more than 21 μm. 2.A method according to claim 1 wherein the frequency modulated halftonedot with the greatest main dimension on said lithographic printing platehas a main dimension of more than 25 μm.
 3. A method according to claim1 wherein the frequency modulated halftone dot with the greatest maindimension on said lithographic printing plate has a main dimension ofnot more than 100 μm.
 4. A method according to claim 1 wherein saidhydrophilic surface of a support is a grained and anodized aluminumfoil.
 5. A method according to claim 2 wherein the main dimension of atleast 75% of the frequency modulated halftone dots is more than 25 μm.6. A method according to claim 5 wherein the main dimension of at least90% of the frequency modulated halftone dots is more than 25 μm.
 7. Amethod according to claim 1 wherein said frequency modulated halftonedot with the greatest main dimension consist of one microdot.
 8. Amethod according to claim 1 wherein said frequency modulated halftonedot with the greatest main dimension consist of a m*n matrix of "microdots" wherein m represents an integer of at least 2 and n represents aninteger of at least 1 and m*P and/or n*P>21 μm and P being the recorder"pitch".
 9. A method according to claim 8 wherein said m and nrepresents the same integer.
 10. A printing plate obtained by the methodaccording to claim 1.